Effects of dietary isoleucine on growth, the digestion and absorption capacity and gene expression in hepatopancreas and intestine of juvenile Jian carp (Cyprinus carpio var. Jian)

The present research studied the effects of dietary isoleucine (Ile) on growth performance, the digestion and absorption capacity, as well as gene expression in hepatopancreas and intestine of juvenile Jian carp (Cyprinus carpio var. Jian). A total of 1200 juvenile Jian carp (Cyprinus carpio var. Jian) (6.9 ± 0.03 g) were randomly distributed into six groups with four replicates each, fed semi-purified isonitrogenous diets (335.8 g crude protein/kg diet) containing graded levels of Ile (4.2, 7.0, 9.5, 11.9, 13.9 and 16.9 g/kg diets) for 60 days. The relative expression of gene was performed by real-time quantitative PCR. Compared with the control group, Ile supplementation increased (P < 0.05): (1) specific growth rate (SGR) and feed intake, (2) body protein content and protein retention value, (3) intestine fold height, (4) activities of trypsin, chymotrypsin, lipase and amylase in hepatopancreas and intestine, (5) activities of alkaline phosphatase in distal intestine, Na+/K+-ATPase and γ-glutamyl transpeptidase in intestine, (6) glutamate–oxaloacetate transaminase activity in hepatopancreas, and (7) relative mRNA expression of chymotrypsin, lipase and amylase in hepatopancreas, γ-glutamyl transpeptidase in mid intestine, Na+/K+-ATPase in intestine and target of rapamycin (TOR) in hepatopancreas and mid intestine. However, Ile supplementation decreased (P < 0.05): (1) feed conversion ratio, (2) glutamate–pyruvate transaminase activity in hepatopancreas, and (3) relative mRNA expression of trypsin in hepatopancreas, alkaline phosphatase in intestine, γ-glutamyl transpeptidase in distal intestine and eIF4E-binding protein (4E-BP) in hepatopancreas, proximal and mid intestine. Collectively, this study indicated that dietary Ile improves fish growth, promotes the digestive and absorptive ability and regulates gene expression of the digestive and brush border enzymes, TOR and 4E-BP. Based on the quadratic regression analysis of SGR, the Ile requirement of juvenile Jian carp (6.90–63.39 g) was estimated to be a 12.9 g/kg diet, corresponding to 38.4 g/kg of dietary protein.

► Optimal levels of Ile exerted a positive effect on the growth performance.
► The effect of Ile on the digestive and absorptive capacity in fish was evaluated.
► Ile regulated the digestive and brush border enzymes, TOR and 4E-BP gene expression.
► The Ile requirement of Jian carp (6.90–63.39 g) was estimated to be a 12.9 g/kg diet.